Vehicle rollback may occur when a vehicle is stopped on an inclined road surface. Hybrid powertrains typically turn off the engine when the vehicle is stopped and restart the engine when the brakes are released. The vehicle rollback may occur between the time that the brakes are released and the time that sufficient output torque is present.
Some conventional powertrain control systems employ a neutral idle control strategy. For this type of vehicle, the transmission is automatically moved to neutral while idling to reduce fuel consumption and/or idle vibration. The vehicle is held on uphill inclines using the vehicle braking system, and hardly at all by the drivetrain. Vehicle rollback may be more pronounced in vehicles with relatively high vehicle mass and relatively low engine torque. In these vehicles, the drive axle is not preloaded with a sufficient amount of torque following brake release while the vehicle is stopped to overcome the effects of gravity. Current solutions include increasing idle speed to increase available drive torque when the brakes are released. However, increasing idle speed requires increased idle speed for all conditions, even when the increased idle speed is not necessary. The increased idle speed reduces fuel economy and increases noise.